The invention relates in the development of aerodynamic flexible cutting string connectable to a rotating head for cutting vegetation, and a vegetation cutting head holding the aerodynamic string in place, and a method of cutting vegetation using a string. In vegetation cutters, such as shown in U.S. Pat. No. 5,761,816 (the disclosure of which is incorporated by reference herein), it is possible to achieve vastly improved cutting results and/or other advantages, in vegetation cutting string, devices, and methods by using an aerodynamic line.
An aerodynamic line (also known as a "string") has a beam structure comprising a strut-like cross section with a first axis which is the strongest axis and has the highest moment of inertia, and a second, weaker, axis. That is, the line has a minimum cross-sectional dimension less than 85% of the maximum cross-sectional dimension, or, for an ellipse, at least a 1.25 or 1.10 to 1 ratio (e.g. between 2:1 to 8:1), and movement in a direction perpendicular to the smaller dimension/axis. Drag-reducing surface texturing can also be provided.
The cross section of an aerodynamic line is typically an elongated shape that may be symmetrical (such as an ellipse or a diamond with rounded apices), or non-symmetrical and/or complex, such as a tear drop, simulating a spearhead comprising a small cross-sectional area rectangle merging into a larger cross-section area trapezoid, simulating a keyway having a smaller diameter circle merging into a large diameter circle, or having a small cross-sectional area trapezoid merging into a larger cross-section area trapezoid of substantially the same shape as the small trapezoid, simulating an ice-cream cone having a small cross-sectional dimension substantially cone-shaped portion merging into a larger cross-sectional dimension substantially parabola shaped portion, simulating a tear drop with an extension nose, or the like.
The aerodynamic cutting line employed in the invention is made up of primarily (more than 50%) or substantially (at least 90%) non-metallic material, such as high strength polymer or other material having similar characteristics. Some examples include nylon, Delrin, polyester, fiberglass, rubber, or rubber-like materials, and mixtures or combinations thereof. Preferably a material that can be heat set into various configurations is provided, such as nylon or nylon mixed with other materials (including fibers or fillers for increased wear resistance or strength).
The aerodynamic cutting line which is utilized in the invention, has a drag coefficient of less than 1.0, typically 0.8 or less, e.g. about 0.35 or less, when rotated about an axis of rotation so that the first axis thereof is positively maintained and moves in a predefined cutting plane, and the rotating string or line is brought into contact with the vegetation so that the cutting line is generally transverse to a portion of the vegetation to be cut, and the string cuts the vegetation. In one example, when the aerodynamic cutting string was asymmetrical and used has a dimension perpendicular to the direction of movement (e.g. the smallest axis) of about 0.08 inches, a vegetation cutter will use less than the equivalent of 0.4 horsepower (per string) at about 8000 rpm for a 17 inch swath, for a 4.5 inch diameter of the rotating head.
It is highly desirable to be able to positively locate the string during cutting action (that is provide pitch control during cutting). This is provided for extruded strings using a cooperating shaped eyelet or the like, or for molded strings by a larger cross-sectional dimension at the head than at the cutting portion of the string, the larger portion either the same or of different shape and with or without an eyelet. Sometimes undesirable stress can be introduced into the cutting line under these circumstances, which can shorten its life and cause it to lose pitch control.
According to the present invention, a flexible cutting line/string is provided which is formed with a twist therein, and is mounted in the head of a vegetation cutter at a portion opposite the twist from the actual cutting portion of the string. The provision of the twist, and mounting inward from the twist (the twist about 90.degree. to the cutting plane), allow excellent pitch control while taking the stress off the aerodynamic cutting line, and additionally allow very simple replacement of the cutting line, and allow the utilization of extruded cutting string (which typically is cheaper and stronger than molded string).
As the line exits the `V` groove of the head outlet, a large vertically oriented "living hinge" is provided and formed up to the twist so that if a solid barrier is impacted, the string can easily deflect with reduced stress on the cutting element, as well as reduction of the string trimmer thrust. In a preferred embodiment two twists are provided in pre-cut length of extruded primarily or substantially non-metallic flexible cutting line, with the string mounted between the two twists.
According to one aspect of the present invention, a method of cutting vegetation using a powered head rotatable about an axis of rotation, and at least one string of primarily non-metallic material having a first long axis and a second short axis, with the second axis less than 85% of the first axis, the string having at least one free end and at least one substantially permanent twist remote from the free end, is provided. The method comprises the steps of: (a) Mounting the string in the head at a first portion of the string so that the string extends radially outwardly from the head with the at least one free end remote from the head and a portion of the string on the opposite side of the free end from the twist operatively engaging the head, while orienting the string so that the first axis is substantially perpendicular to the axis of rotation. And, (b) powering the head about the axis of rotation so that the first axis is substantially maintained in a cutting plane substantially perpendicular to the axis of rotation, the living hinge and twist providing pitch control while allowing deflection of the string when impacting an object substantially incapable of being cut.
The head may include a groove or slot having a receiving dimension about at least as wide as the second axis but not as wide as the first axis. In this case step (a) is practiced so that the second axis of the string portion opposite the twist from the free end is received by the groove or slot which has its long axis generally perpendicular to the axis of rotation.
Typically the string has two twists therein, with a portion of the string between the twists, and step (a) is practiced to mount the portion of the string between the twists to the head. The head may be configured, and step (a) practiced, so that the free ends of the string are typically either about 90.degree., or about 135.degree., or about 180.degree., apart during the practice of step (b). Step (a) may in addition to using the groove or slot (or in place thereof) be practiced by clamping the string to the head, or other techniques may be utilized, such as adhesive tape, a material which solidifies but may be removed from the head by application of heat, a solvent, or the like, etc. The groove or slot may be non-linear and step (a) may be practiced by mounting the string so that it extends in the nonlinear groove or slot.
Utilizing the method of the invention it is possible to achieve effective cutting by rotating the head (and a tangential velocity of a free end of the string) at a slower speed than in conventional vegetation trimmers, thus saving on fuel (e.g. gasoline) or electrical energy (if the vegetation cutter is battery powered, or connected by a cord to a 120 volt source). Therefore step (b) may be practiced so that the free end of the string moves at a tangential velocity at least 5% lower (e.g. about 10%, or even more, lower) than the tangential velocity of the free end of a conventional round cross-section nylon string powered by the head, yet with the same or enhanced cutting action.
According to another aspect of the present invention a string primarily (or substantially) of non-metallic material (such as nylon), and either extruded or molded (but preferably extruded) is provided. The string has: a first long axis and a second short axis, with the second axis less than 85% of the first axis; a length of between about 5-25 inches; a first free end; a second end; and, a permanent living hinge and twist between the first and second ends.
Preferably the twist in the string is between about 30-150.degree. (e.g. about 90.degree.) so that the first axis on one side of the twist makes an angle with respect to the first axis on the other side of the twist of between about 30-150.degree. (e.g. about 90.degree.). Preferably the second end is also a free end, and there is a second living hinge and twist between the first and second ends, and a central portion between the first and second hinges. The central portion may be linear, or could be formed in a pre-determined non-linear shape (such as an S-shape for cooperation with a S-shaped groove or slot that may be asymmetrical or symmetrical). The string may have a wide variety of cross-sections, symmetrical or non-symmetrical (such as substantially a tear drop cross-section when non-symmetrical, or a substantially elliptical cross-section when symmetrical, with the first axis between 1.25-8 times as long as the second axis).
The string is typically in combination with the head of a vegetation cutter, the head rotatable about an axis, and having a retaining mechanism (such as a clamp, groove or slot, or the like as described above) which holds the string (e.g. engaging pre-formed flats and/or twists in the string) to the head on the opposite side of the twist from the free end of the string so that the free end is remote from the head, and so that the first axis of the string is substantially perpendicular to the axis of rotation of the head. Where the string has two twists in a central portion, the central portion is held by the head so that the free ends are remote from the head and so that the first axis of the string between each twist and the free end is substantially perpendicular to the axis of rotation. The string may be held to the head by a slot or groove and the head may have a string-receiving opening with a width substantially the same as or greater than the second axis but less than the first axis. The first and second free ends may be spaced approximately 90.degree., or approximately 180.degree., or other angle, apart during rotation of the head. A plurality of strings may be associated with any particular head.
The invention also relates to a vegetation cutter comprising the following components: A head, handle, and motor. A substantially non-metallic cutting string having at least a first free end, the capability of a drag coefficient of less than 0.8, a living hinge and twist, and a mounting portion opposite the free end from the twist. And, the cutting string mounting portion mounting the cutting string to the head so that the first free end is remote from the head and the cutting string has a drag coefficient of less than 0.8 when rotated by the head. The details of the cutting line, mount to the head, etc., may be as described above, and the drag co-efficient may be as low as about 0.35, or even less. The cutting string may be in a cartridge, and the head may mount the cartridge so that it is reversible, that is can be mounted with either face of the cartridge up or down.
According to another aspect of the present invention a method of making string for a vegetation cutter is provided. The method preferably comprises: (a) heating a section of a primarily non-metallic material string to at least its softening point; (b) twisting portions of the string on opposite sides of the heated section of (a) with respect to each other to introduce a twist into the string; and (c) cooling the string heated section to well below the softening point thereof while the twist is held therein so that the twist becomes permanently fixed in the string.
Preferably (a)-(c) are practiced with string that is non-round or non-symmetrical in cross section, and (b) and (c) are practiced to produce a twist of between 30-150.degree. (preferably about 90.degree.); and the method further comprises (d) using the string in a vegetation cutter. Preferably (d) is practiced by rotating the string at a slower speed (e.g. about 2500-7500 rpm, preferably about 3500-6500 rpm) than when using round, untwisted, symmetrical string with a vegetation cutter of the same size and type.
It is the primary object of the present invention to provide a highly advantageous cutting string, which when used in combination with a vegetation cutting head or in a method of cutting vegetation, has numerous advantageous results. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.